A heat pump raises the temperature level of heat by means of work input. The pump cycle is identical with the vapor compression refrigeration system. It finds particular application for air-conditioning of an air space such as a home since it employs the same equipment to cool the conditioned space in the summer and to heat it in winter. This dual purpose is accomplished by valving which places the low temperature evaporator in the conditioned space during the summer and the temperature condenser in the same space during the winter. In effect, such heat pumps heat (or cool) the interior of the buildings by refrigerating (or heating) the outdoors. The principle of this operation was first described by Kelvin in 1852.
The coefficient of performance cp.sub.c for cooling a conditioned air space is given in equation (1) and the coefficient cp.sub.w for warming the space is given in equation (2), wherein T is the temperature in absolute degrees and the subscripts c and h refer to the cold and hot temperatures, respectively. ##EQU1##
It is apparent from the foregoing equations that maximum performance is obtained when the temperature differential between the outside temperature and that of the conditioned space is a minimum.
The most common source of exterior heating or cooling is the surrounding air because of its convenience. However, since the air heats to relatively high temperatures in the summer and cold temperatures in the winter, it is the least efficient source of cooling and heating. Furthermore, in most temperate zones, the heating load is usually greater than the cooling load. This leads to an imbalance in the sizing of equipment and necessitates a large, high horsepower compressor fitted to the heat demand, a supplementary heating system (electrical resistance or fuel), or a heat-storage system.
The use of well water as a heat source is more efficient than atmospheric air. However, the impurity, quality, quantity and disposal of water and the corrosion problems of the pipe and the water have minimized the use of such systems.
Another heat source is the use of the earth itself by laying a heat pipe of the heat pump in an underground hole and then backfilling it with soil. The earth is potentially the most desirable heat source or sink because of its availability and seasonal uniformity of temperature. However, heat transfer between the liquid in the heat pipe and the soil depends largely upon the moisture content which is related to climate conditions and geological formation. That is, as the moisture content decreases, the thermal conductivity of the soil and thus the efficiency of heat transfer with the liquid in the heat pipe correspondingly decreases. This is a particular problem during the summer months in which the moisture content decreases to a minimum, due, in large part, to the heating of the soil by solar energy during the longer daytime periods.
Recently, various hydrophilic polymeric gel substances have been developed with extremely high water holding capacities. One such product is described in an article entitled "Super Slurper-Compound with a Super Thirst," Agricultural Research, June 1975 (published by Agricultural Research Service, USDA). It is a hydrolyzed starch polyacrylonitrile graft copolymer. One use disclosed for this material is to increase the water-holding capacity of sand to enhance the top growth of crops such as oats. The article states that the sand, by itself, retains only 24 grams of water compared with 317 grams of water held by the sand-gel mixture at a concentration of one part of gel to 250 parts of sand. Such gels are said to absorb as high as 1,000-2,000 times their weight of water.
Another type of hydrophilic gel is sold by Union Carbide under the trademark "Viterra" hydrogel. This synthetic material is suggested by Union Carbide to be used as an additive to the soil to assist transfer of water and nutrients to a growing plant. Product literature from Union Carbide suggests that the Viterra hydrogel can retain more than twenty times its dry weight of water. Another such product called "Imbiber Beads" is manufactured by the Dow Chemical Company with a 27:1 holding capacity.
All these polymers have the capacity to take in a large quantity of water without becoming dissolved. The water actually penetrates the polymer network causing the size of the particle to increase, but in so doing, no large pockets of water are formed which might later leak out. The water is actually entrapped by the molecular structure of the polymer. It is extremely difficult to squeeze out entrapped water from the polymer. However, water can be evaporated from the polymers, and the starch-based copolymer is biodegradable.